Apparatus for removing sulfur dioxide from stack gases

ABSTRACT

Method and apparatus for removing sulfur dioxide from stack gases. The stack gases are scrubbed with an aqueous wash liquid containing one or more alkali metal compounds which react to form the bisulfite, sulfite and sulfate salts of the alkali metal. The spent absorbent liquid effluent is then treated with calcium compounds in at least two separate reaction stages, the conditions of ion concentration and pH of the first stage being maintained to favor the precipitation of calcium sulfate in the presence of SO 3   =  ions. In the succeeding reaction stages the precipitation of the desired amount of SO 3   =  ions is carried out to form calcium sulfite. The resulting compound of calcium with sulfite and sulfate (which may in part be in the form of mixed crystals) are removed and the regenerated liquid is recycled.

This application is a division of application Ser. No. 424,212 filedDec. 13, 1973, now U.S. Pat. No. 3,944,649.

This invention relates to a process for the removal of sulfur dioxidefrom stack gases and more particularly to such a process wherein stackgases are contacted with a wash liquid to form an alkali metalbisulfite, sulfite and sulfate and the alkali metal ion values aresubsequently recovered for recycling.

With the more recent realization for the need for clean air, it hasbecome necessary to maintain the sulfur dioxide content in stack gasesbelow prescribed minimum levels. The two obvious alternative approachesto attain this goal are the use of essentially sulfur-free fuel and theprocessing of the stack gases to remove sulfur dioxide resulting fromthe use of sulfur-containing fuels. Since, however, sulfur-free fuelsare generally more expensive than sulfur-containing fuels and are,moreover, not available in sufficient quantities for many large-scaleuses (e.g., electric utility and industrial scale boilers) it isnecessary to provide efficient and economical means for removal ofsulfur dioxide from stack gases.

Among some of the approaches being considered suitable for removal ofsulfur dioxide in stack gases from utility- and industrial-scale boilersare (1) once-through scrubbing with a solution of sodium carbonate orsodium hydroxide; (2) lime or limestone slurry scrubbing; and (3) sodiumscrubbing with lime regeneration. (See "Chemical Engineering ProgressTechnical Manual, Sulfur and SO₂ Developments" A.I.Ch.E. pp 142-150(1971).)

Once-through sodium scrubbing represents, at least for some situations,the simplest and most reliable process; but its application is limitedto locations where the dissolved solids load, when combined with theremainder of the plant liquid effluent, can be treated to be acceptableas a liquid waste stream. Direct lime or limestone slurry scrubbingproduces a low-solubility solid waste and is applicable at a wider rangeof locations than once-through sodium scrubbing. Although the cost ofscrubbing chemicals is relatively low, capital cost requirements arehigh because a slurry of solid calcium salts must be recirculated athigh rates, and the attainment of reliability is still in doubt becauseof the possibility of the uncontrolled deposition of a solid scale invarious parts of the system.

Sulfur dioxide removal based upon sodium scrubbing with lime and/orlimestone regeneration incorporates the better features of the first twoprocesses and offers additional advantages over both of them. Thus, itmay be shown that sodium scrubbing with lime regeneration can use lessexpensive raw chemicals, produce a solid waste, can be designed forhigher removal efficiency, can minimize or even eliminate scalingproblems, and can lower investment costs and operating and maintenanceproblems.

In the process based upon sodium scrubbing with lime regeneration, thestack gases are scrubbed with an aqueous solution of sodium hydroxideand sodium sulfite (and perhaps some makeup sodium carbonate) to produceadditional sodium sulfite and sodium bisulfite. Because the stack gasesnormally contain at least a few percent oxygen, a portion of the sodiumsulfite is oxidized to sodium sulfate. This in turn means that the spentabsorbent liquid effluent from the scrubber contains sodium bisulfite,sodium sulfite and sodium sulfate. In order to recover the sodium valuesfrom these compounds it is necessary to precipitate compounds of calciumwith sulfite and sulfate. But in the presence of reasonably high andeconomically feasible concentrations of SO₃ ⁼ ions, calcium sulfate isnot formed which means that sodium values are lost and the sodiumsulfate concentration in the recirculating liquid continues to increase.One obvious solution to this problem of sodium sulfate build-up is tooperate with scrubbing liquids and effluents having SO₃ ⁼ concentrationswhich are sufficiently low to permit precipitation of the SO₄ ⁼ ascalcium sulfate. This solution, however, gives rise to high Ca⁺ ⁺ ionconcentrations in the regenerated liquor which in turn requires furtherprocessing to soften the liquor and to remove CA⁺ ⁺ ions in order toprevent scaling in the scrubber. Moreover, operating the process withlow SO₃ ⁼ concentrations requires excessively large equipment. In orderto realize all of the advantages associated with the process of sulfurdioxide removal based upon sodium scrubbing with lime regeneration, itwould be desirable to be able to operate in a concentrated sodiumsulfite mode using a reasonably-sized plant and a minimum quantity ofrecycling liquid with low CA⁺ ⁺ ion concentration by providing for theefficient removal of the SO₄ ⁼ present in the scrubber effluent.

It is therefore a primary object of this invention to provide animproved process for the removal of sulfur dioxide from stack gases, theprocess incorporating sodium scrubbing with lime and/or limestoneregeneration. It is another object to provide a process of the characterdescribed which achieves the effective removal of So₄ ⁼ ions from thespent liquid effluent as calcium sulfate, or calcium sulfate/calciumsulfite mixed crystals, and the return of the sodium values derived fromsodium sulfate into the recycled liquid. It is yet another object ofthis invention to provide such a process which is relatively simple andwhich can be effectively and efficiently carried out with the minimum ofcomplexity, equipment size and stream flow rates. Still another objectof this invention is to provide a process for removing sulfur dioxidefrom stack gases which permits operation with high HSO₃ ⁻ ionconcentrations, which makes it possible to provide at least a portion ofthe Ca⁺ ⁺ ions from limestone rather than from the more expensive lime;and which does not require any adjustment of the temperature of thescrubber effluent prior to the recovery of the sodium values. It is anadditional object to provide a process for removing sulfur dioxide fromstack gases which results in the formation of precipitated solids havinggood settling and filtration characteristics thereby eliminating anysignificant carryover of suspended calcium salts back into the scrubber,and which also provides a high sulfite ion concentration in the reactorsystem which in turn gives rise to low calcium ion concentrations in theliquor.

Other objects of the invention will in part be obvious and will in partbe apparent hereinafter.

In the process of this invention sulfur dioxide in stack gases isremoved by reaction with one or more water-soluble sodium compounds toform sodium bisulfite, sodium sulfite and sodium sulfate. The scrubbereffluent liquid containing the reaction products is then reacted withcalcium compounds in multiple stages, the reaction conditions of thefirst of these multiple stages being maintained to favor theprecipitation of calcium sulfate in the presence of a relatively highconcentration of SO₃ ⁼. Some calcium sulfite is formed in the firststage and the remaining required amount is precipitated out in thesucceeding stage or stages. Thus a steady state condition is establishedin the system whereby the SO₄ ⁼ ions are removed as formed even in thepresence of a relatively large SO₃ ⁼ ion concentration.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and theapparatus embodying features of construction, combination of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure, and the scope of theinvention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the single accompanying drawing which is a flow diagramof the method of this invention.

In the following description, it will be assumed for convenience indescribing the invention that sodium compounds, e.g., NaOH and Na₂ SO₃are used in the scrubbing liquid. However, it is to be understood thatthe hydroxides, carbonates and sulfites of the other alkali metals(i.e., lithium, potassium, rubidium and cesium as well as ammonium) maybe used in the process of this invention.

The process of this invention may be further described in detail withreference to the drawing which is a somewhat simplified flow diagram.The stack gases from which sulfur dioxide is to be removed areintroduced into a scrubber 10 which is a mass transfer apparatussuitable for effecting efficient gas-liquid contact to react therequired amount of the SO₂ with the sodium hydroxide and sodium sulfitesupplied as an aqueous solution through line 11 from a wash liquidsource 12 by a pump 13. The scrubber 10 may be any suitable commerciallyavailable apparatus such as a countercurrent or cocurrent scrubbingdevice or combination of such devices. In the drawing, a countercurrentscrubber is shown in which the SO₂ - containing stack gas is introducednear the bottom through line 14 and the SO₂ -lean gas is discharged nearthe top through line 15. It will be appreciated that it will normallynot be feasible to remove all of the SO₂ from the stack gases and thatthe actual amount of SO₂ to be removed will be preset from any one givenset of circumstances and that the operational parameters of the SO₂-removal process will then be determined to meet the requirements of thesystem designed for the circumstances.

Within scrubber 10 the absorption of SO₂ gives rise to the formation ofsodium bisulfite, sodium sulfite and sodium sulfate according to thefollowing overall reactions

    2NaOH + SO.sub.2 → Na.sub.2 SO.sub.3 + H.sub.2 O    (1)

    na.sub.2 SO.sub.3 + SO.sub.2 + H.sub.2 O → 2NaHSO.sub.3 (2)

    2na.sub.2 SO.sub.3 + O.sub.2 → 2Na.sub.2 SO.sub.4.  (3)

this solution, which is the spent absorbent liquid effluent, iswithdrawn through line 16 into a reactor system 17 which is seen toconsist of multiple stages, e.g., 18 and 19. Neutralization of thissolution by the addition of lime (Ca(OH)₂) and/or limestone (CaCO₃)readily forms sodium sulfite and/or sodium hydroxide

    Ca(OH).sub.2 + 2NaHSO.sub.3 → CaSO.sub.3 + Na.sub.2 SO.sub.3 + 2H.sub.2 O                                                (4)

    caCO.sub.3 + 2NaHSO.sub.3 → CaSO.sub.3 + Na.sub.2 SO.sub.3 + CO.sub.2 + H.sub.2 O                                      (5)

    ca(OH).sub.2 + Na.sub.2 SO.sub.3 → CaSO.sub.3 + 2NaOH (6)

however, the reaction

    Ca.sup.+.sup.+ + SO.sub.4 .sup.= → CaSO.sub.4       (7)

(whether producing CaSO₄ as such or a mixture of CaSO₄ and CaSO₃crystals) required to precipitate CaSO₄ in some form and free the sodiumvalues in Na₂ SO₄ for active participation in the absorption processdoes not take place in the presence of relatively high sulfiteconcentrations. We have found, however, that a sufficient quantity ofthe sulfate ions present can be removed by carrying out thecausticization, or neutralization, in two or more stages provided theconditions in the first stage 18 are maintained to favor reaction (7),that is, favor the formation of CaSO₄. In the following description andin the claims defining our invention, it is to be understood thatprecipitation of CaSO₄ includes the precipitation of mixed saltscontaining CaSO₄. These conditions in the first stage to produce CaSO₄are generally concerned with ion concentration, pH and residence time inreactor 18. The temperature of the reactor has little, if any, influenceon the reactions and hence is not one of the conditions. This in turnmeans that no adjustment in the temperature of the spent absorbenteffluent delivered to reactor 18 is necessary. In general, thetemperature of the scrubber liquid effluent will range between about110° and 130° F.

The conditions of ion concentrations which should prevail in the feed tothe first stage may be expressed in concentration of total oxidizablesulfur (HSO₃ ⁻ + SO₃ ⁼), SO₄ ⁼ ion concentration and the ratio of HSO₃ ⁻/SO₃ ⁼. Oxidizable sulfur concentration may be as high as about onemolar, and it is desirable to work with concentrations approaching thislevel to maintain equipment size and cost as low as possible. However,total oxidizable sulfur concentrations may be as low as about 0.1 molar.Sulfate ion concentrations in the first stage may range between about0.1 and 2.8 molar and will depend to a great extent upon the quality ofthe stack gases, i.e., the extent to which they are capable of oxidizingsome of the Na₂ SO₃. The ratio of HSO₃ ⁻ /SO₃ ⁼ may range from ∞ to 1,that is, this ratio should be at least one.

The quantity of lime and/or limestone added to the first stage reactor18 is preferably that which is less than sufficient to attain fullneutralization. Thus the pH maintained in the first stage reactor shouldbe on the acid side to obtain optimum results, and more specifically, itshould range from about about 4.5 and 6.8. However, some of the benefitsof the two-stage reactor system of this invention (attainment of goodsettling and filtering characteristics and some sulfate regeneration)may be realized by operating the first stage well beyond the neutralcondition to higher pH's provided the residence time in reactor 18 isproperly adjusted. By carrying out this first stage of neutralization sothat the spent absorbent liquid under treatment remains acidic,CaSO₄.sup.. xH₂ O is precipitated and can be subsequently removed bysettling and/or filtration techniques. Since the CaSO₄.sup.. xH₂ Ocrystals thus formed are subject to redissolving as equilibrium withcalcium sulfite and sulfate is approached, the residence time of theprecipitated solids in the first stage reactor should be short enough tominimize this redissolution. This means that residence time of thesolids in the first stage reactor should be no greater than about 30minutes and preferably should be from about 3 to 5 minutes. Whenlimestone rather than lime is used, the longer residence time may bemore desirable.

By maintaining the above-specified conditions in the first stage reactor18, CaSO₃.sup.. yH₂ O crystals are formed along with CaSO₄.sup.. xH₂ Ocrystals, and these crystals have good settling and filtrationcharacteristics which make it possible to easily remove them from theregeneration effluent slurry. As will be seen in the drawing, a portionof the slurry from the first stage reactor 18 is taken directly to asolid/liquid separator 20 while the remainder of the slurry isintroduced into the second stage reactor 19 for complete neutralizationwith additionally supplied lime and/or limestone. The crystals formed inthe first stage reactor and carried over into the second stage reactorserve as seed crystals. In any case, the sodium bisulfite and a majorportion of the sodium sulfite is finally reacted with the lime and/orlimestone in the second stage reactor 19 to form the desired Na₂ SO₃ andNaOH according to reactions (4) - (6). A sufficient quantity of limeand/or limestone is added to the second stage reactor to continueneutralizing it or to make it basic, i.e., to give it a pH up to about12 when using lime. Under these conditions the remaining required amountof the CaSO₃ will precipitate out; and the major portion of the solidsformed will be CaSO₃. Although only one multistage reactor system isshown in the drawing, it is within the scope of this invention to havemore than one reactor system in parallel so that there can be severaltrains of multistage reactors. The actual number of reactors will dependupon the required sulfur removal capacity. Under some conditions, theeconomics or the cost of lime and limestone may justify more than tworeactor stages in series in order to keep down the size of the reactorsystem and to optimize the utilization of the chemicals. Thedetermination of the advisability of using more than one second stagereactor, as well as the use of parallel reactor systems, is well withinthe skill of the art, and will depend upon such factors as flow rateshandled, equipment used, space available and the like.

The CaSO₃ formed in the second stage reactor, as well as the mixed CaSO₄/CaSO₃ product formed in the first stage reactor, are separated out inthe separator system 20 from which the solid phase containingCaSO₃.sup.. yH₂ O and CaSO₄.sup.. xH₂ O is discharged for discard orfurther processing and the liquid phase containing Na₂ SO₃ in solutionis directed to scrubber feed tank 12 into which makeup sodium values,normally, as sodium hydroxide or carbonate, are added.

As shown in the drawing, an auxiliary first stage reactor 18a may beused in addition to first stage reactor 18 to handle a portion of thespent absorbent effluent. The reaction conditions in this auxiliaryfirst stage reactor are the same as previously described for reactor 18.All of the slurry formed in auxilliary reactor 18a, comprising liquidand crystals of CaSO₄ and CaSO₃, is transferred directly to thesolid/liquid separator system 20. The use of the auxiliary first stagereactor makes it possible to handle a slip stream for sulfateregeneration which can be processed separately for solid-liquidseparators and thus to minimize the amount of CaSO₄ which may beredissolved.

The reactors may be any suitable commercially available apparatusdesigned to handle this type of reaction system. Such reactors include,but are not limited to, baffled, stirred tank reactors. The solid/liquidseparator may likewise be any suitable commercially available equipmentsuitable for separating solids from a liquid slurry. Examples of suchseparators are thickeners, clarifiers, filters, centrifuges orcombinations of these. Separate separators may be used for the first andsecond stage reactor effluents to minimize the amount of CaSO₄ which mayredissolve.

It will be seen from the above detailed description that the process ofthis invention makes it possible to provide solid regeneration productscomprising both CaSO₄.sup.. xH₂ O and CaSO₃.sup.. yH₂ O having settlingand filtration characteristics such that the solids can be easilyremoved from the effluent slurry to provide a regenerated wash liquidwhich has an acceptably low and constant concentration of Na₂ SO₄. Thusthe ion concentrations at the various reaction stages of the process aremaintained within optimum ranges and maximum sodium values arerecovered. Moreover, the process of this invention makes it possible towork with relatively high SO₃ ⁼ and HSO₃ ⁼ ion concentrations in thewash liquid which in turn makes it possible to keep down the size of theequipment used. By carrying out only a partial neutralization in thefirst reactor stage, the cheaper limestone may be used in place of or asa substitute for a part of the lime used, thus contributing to theeconomics of the process.

Because the process generates precipitated solids which have goodsettling and filtration characteristics, thus eliminating carryover ofany significant amount of calcium salts into the scrubber, and becausethe high sulfite concentration in the reactor system provides for a verylow Ca⁺ ⁺ ion concentration in the liquor, the Ca⁺ ⁺ ion concentrationis very low in the regenerated scrubber liquor which in turn means thatno additional chemicals or equipment must be supplied to soften theregenerated liquid to prevent scaling.

Finally, since the process is not influenced by fluctuations intemperature within the ranges normally encountered in scrubbing stackgases, there is no need to provide means for effecting a temperatureadjustment in the spent absorbent effluent discharged from the scrubber.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above process andin the constructions set forth without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing shall be interpreted asillustrative and not in a limiting sense.

We claim:
 1. An apparatus for removing sulfur dioxide from stack gases, comprising in combination(a) mass transfer means for contacting SO₂ -containing gases with a SO₂ -absorbent solution containing at least one alkali metal compound and including means to separately withdraw SO₂ -lean gases and spent absorbent wash-liquid effluent containing reactant bisulfite, sulfite and sulfate salts of said alkali metal; (b) first stage reactor means for reacting said salts of said alkali metal in said spent wash liquid effluent under conditions to favor the precipitation of sulfate ions as calcium salts in the presence of sulfite ions along with the precipitation of some sulfite ions as calcium salts to form a first solids/liquid slurry;(c) means to introduce a controlled amount of at least one calcium compound into said first stage reactor; (d) a second stage reactor separate from said first stage reactor for reacting a portion of said slurry from said first stage reactor under conditions to complete the precipitation of a predetermined amount of said bisulfite and sulfite alkali metal salts as calcium sulfite to form a second solids/liquid slurry; (e) means to introduce a controlled amount of at least one calcium compound into said second stage reactor; (f) solids/liquid separator means adapted to separate solids from said first and second slurries; (g) first slurry transfer means arranged to transfer a portion of said first slurry from said first stage reactor directly to said second stage reactor and the remainder of said first slurry from said first stage reactor directly to said solids/liquid separator means; (h) second slurry transfer means arranged to transfer said second slurry from said second stage reactor to said solids/liquid separator means; and (i) means to recycle liquid from said solids/liquid separator means as at least a portion of said SO₂ -absorbent wash liquid.
 2. An apparatus in accordance with claim 1 including(j) auxiliary first stage reactor means for reacting said salts of said alkali metal in said spent wash liquid effluent under conditions essentially the same as in said first stage reactor means to form a third slurry; (k) means to divide said spent absorbent wash liquid effluent from said mass transfer means into first and second streams; (l) means to direct said first stream to said first stage reactor means and said second stream to said auxiliary first stage reactor means; and (m) third slurry transfer means arranged to transfer said third slurry from said auxiliary first stage reactor directly to said solids/liquid separator means. 